1. Field of the Invention
This invention pertains to speech processing and, more particularly, to digital speech processing in a delta modulation environment.
2. Description of the Prior Art
Speech processing, which includes speech recognition and synthetic generation of speech, is most commonly carried out with digital means because, generally, speech processing requires considerable amount of processing and storage. Digitally encoded speech, however, contains quantization noise which results from the quantizing process, and also, digitally encoded speech lacks the simplicity with which amplitude, frequency, noise, or silence can be ascertained.
In speech recognition applications, detection of silence periods between utterances is obviously important for recognizing the speech since the instants of utterance beginnings and endings must be known. Additionally, detection of silence periods aids in reducing the storage and processing burden because no processing is required to be performed during silence periods. In An Algorithm for Locating the Beginning and End of an Utterance Using ADPM Coded Speech, BSTJ Volume 53, No. 6, July-August, 1974, pages 1127-1135, Rosenthal et al. describe a method for detecting silence periods in an Adaptive Differential Pulse Code Modulation (ADPCM) system. The described method utilizes the fact that when silence occurs in an ADPCM encoded system, the step size seeks its minimum value and the resulting code words vary only slightly. This fact is embodied in the described test that evaluates a "code word energy" function, which is defined as the sum of squares of the code words over a 101 sample window centered about a center sample. The evaluated energy function is compared to a threshold and, based on the threshold decision, a determination as to the existence of silence is made.
In applications where speech is encoded, stored, and reconstituted for speech synthesis purposes, signal behavior during silence periods is of additional concern because system noise is most noticeable during silence periods. This noise, called idle channel noise, predominantly originates from the time and amplitude truncation of the digitizing operation and from the imperfections of the encoder. A perfect delta modulator, for example, would respond to a perfect silence input signal (0 volts DC) by developing a bit sequence of alternating "1"s and "0"s. In actual delta modulation encoders, the ideal bit pattern is not achieved but is interspersed with random sequences of consecutive "1"s and/or "0"s. The auditory result of the consecutive "1"s and/or "0"s is noise.
In adaptive delta modulators, where the step size is dependent on the sequence of output bits, limit cycles are sometimes established during silence periods, and those limit cycles, when decoded, develop an annoying hum.